Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory, including volatile and non-volatile memory.
Volatile memory requires power to maintain its data, and includes random-access memory (RAM), static random-access memory (SRAM), dynamic random-access memory (DRAM), or synchronous dynamic random-access memory (SDRAM), among others.
Non-volatile memory can retain stored data when not powered, and includes flash memory, read-only memory (ROM), electrically erasable programmable ROM (EEPROM), erasable programmable ROM (EPROM), resistance variable memory, such as phase change random-access memory (PCRAM), resistive random-access memory (RRAM), or magnetoresistive random-access memory (MRAM), among others.
Each type is advantageous in specific settings. For example, DRAM, typically comprising one transistor and one capacitor per bit, is structurally very simple in contrast to other memory types (SRAM, etc.), and as such, is widely used in applications requiring low cost or high capacity. In contrast, SRAM, typically comprising four to six transistors per bit, is faster than DRAM, and is used in applications where speed is a greater concern than cost. However, it can still be advantageous to increase the speed of DRAM operation, such as by using a precharge circuit.
FIG. 3 illustrates a prior art precharge circuit 300 including a first transistor 334 to selectively couple data-line pairs (DL_T, DL_B), and second and third transistors 336, 338 to selectively couple the data-line pairs (DL_T, DL_B) to a bit-line voltage reference (VBLR) (e.g., VARY/2). The precharge circuit 300 further includes fourth and fifth transistors 340, 342 configured to receive a precharge command signal (PRE1) and to provide a precharge voltage (PRE_B) to the first, second, and third transistors 334, 336, 338.
FIG. 4 illustrates prior art operational signals 400 of the precharge circuit 300 of FIG. 3. When PRE1 is high, PRE_B is low, and vice versa, between an overdrive voltage (VOD) and a deactivation voltage (VKK) (e.g., a word-line non-select or deactivation voltage), respectively. The VOD is a voltage above a supply voltage (VARY), commonly used to amplify bit lines during sense operations, and to control the first transistor 334 (an NMOS, precharge transistor). However, if a voltage greater than the gate oxide withstand voltage is applied to a gate of a transistor, the gate oxide of the transistor can breakdown and fail.
When the fourth transistor 340 (a PMOS transistor) coupled to VOD turns on to increase a level of a control signal to the first, second, and third transistors 334, 336, 338, a voltage difference between a gate and a source of the first transistor 334 (an NMOS, precharge transistor) becomes approximately VOD (e.g., the gate of the third transistor 338 is VOD, and the source of the third transistor 338 is DL_B (e.g., VSS, 0V)). If the VOD approaches or exceeds the gate oxide withstand voltage, even if only for a short period of time, the gate insulating film of the first transistor 334 can be destroyed. Accordingly, to protect the transistors of the precharge circuit 300, the VOD must not approach or exceed a gate oxide withstand voltage (e.g., 1.5V) of the transistors.